Starting from Sept. 5, new T-Mobile customers can purchase any smartphone in T-Mobile's lineup of devices or bring their own compatible smartphone and have access to a worry-free unlimited data experience on T-Mobile's nationwide 4G network. Current T-Mobile customers on Classic or Value plans can upgrade their existing service by adding an Unlimited Nationwide 4G Data plan.

The Unlimited Nationwide 4G Data plan will cost $20 per month when added to a Value voice and text plan or $30 per month when added to a Classic voice and text plan. For example, a single line Value plan with unlimited talk and text combined with unlimited nationwide 4G data will cost $69.99 or a single line Classic plan with unlimited talk, unlimited text and unlimited nationwide 4G data will cost $89.99.

T-Mobile USA had stopped offering such plans early last year.

The move comes as T-Mobile's three larger rivals are thought to be gearing up to begin selling the iPhone 5 in a month or so.

No. 3 U.S. carrier Sprint Nextel already has an unlimited-data plan and credits that with helping it attract customers for its smartphones.

AT&T, the nation's second-largest carrier, stopped signing up customers for unlimited data plans two years ago, while No. 1 Verizon Wireless stopped last year. Both have shifted toward lifting all limits on calls and texting, but limiting data usage.

T-Mobile's new plan will cost $5 less than a regular data plan with 5 gigabytes of full-speed data. The advantage of the limited plan is that subscribers can turn their phones into "mobile hotspots," linking tablets and computers to the Internet through the phone. That's not allowed under the unlimited plan.

T-Mobile has upgraded its network to higher speeds and is losing customers, in large part because it doesn't sell the iPhone.